Groundwater Assessment in Apapa Coast-Line Area of Lagos Using Electrical Resistivity Method

Transcription

1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (4): Scholarlink Research Institute Journals, 2011 (ISSN: ) jeteas.scholarlinkresearch.org Groundwater Assessment in Apapa Coast-Line Area of Lagos Using Electrical Resistivity Method 1 V. C. Ozebo and 2 S.O. Ajiroba 1 Department of Physics, University of Agriculture Abeokuta, Nigeria 2 Department of Physics, Olabisi Onabanjo University Ago-Iwoye, Nigeria Corresponding Author: V. C. Ozebo Abstract A study was conducted on some physical characteristics of geological formations in Apapa, a coast-line area of Lagos state with the purpose of determining the extent of deterioration in the quality and quantity of the groundwater due to saline water intrusion and other factors. Applying the Schlumberger configuration of Vertical electrical sounding, (VES) on five locations, data were collected and analysed using both the partial curve matching and microprocessor iterative techniques. Geoelectric sections along XX XX, comprising of VES point 3 and 1, and along YY YY made up of VES points 1 and 4 were generated along south-north and West-East directions respectively. The analysed data revealed the presence of sand, clay, sandy clay, clayey sand formations and some intrusions of saline water in the area. The results also indicated that the safest depth for groundwater (devoid of saline intrusion) is 34 m and beyond. Keywords: coast-line, deterioration, vertical electrical sounding, saline water, apapa. I TRODUCTIO quality in Apapa area of Lagos state and to see if Deterioration in groundwater quality and quantity in there are traces of saline water intrusions in the area. Apapa area of Lagos state is predominantly caused by drilling hole through an aquiclude which does not quality in Apapa area of Lagos state and to see if contain water at all or that yields water at a very low there are traces of saline water intrusions in the area. rate. When boreholes are drilled in this region, their lifespan are always very short i.e. natural degradation of groundwater is certain to occur. Naturally, sedimentary rocks are predominantly composed of sands, sandstones, clays and limestone. The sedimentary rocks occur in basins, most of which are continuous and extend to neighboring countries. Lagos State lies solely within the Dahomey Sedimentary Basin and has no basement outcrop. The basin extends almost from Accra in Ghana through the republic of Togo and Benin to Nigeria where it is separated from the Niger Delta Basin, (Figure 1, after Oseni, 2005). Lagos state is located in the south western part of Nigeria on the narrow coastal plain of Benin. It lies approximately on longitude E and E and latitude N and N. The study area, Apapa is a coast line area in Lagos state, with major access roads (figure 1b), where most parts are re-claimed (sand filled) for many purposes. The clay and sand layers are not fine in texture as the materials used in filling the area has made it so coarse, dark or brownish in colour, lumpy, rough and fully of odour. The objective of the study is therefore to evaluate the extent of deterioration in groundwater quantity and The purpose of the study is to have enough background information on the extent of deterioration and the intrusions and make adequate and competent recommendations to the appropriate authorities accordingly. 673

2 SURVEY METHODOLOGY The electrical resistivity method of geophysical prospecting is an excellent approach with developed analytic techniques for identifying depth to beds, stratigraphy and lithology as well as for ground water exploration (Lowrie, 1991). It reflects information about resistivity variation with depth. Specially, the Schlumberger electrode array/configuration is adopted in this study as it rates highest in terms of signal response for a given current; also the receiving electrode separation is a large fraction of the current electrode separation (Telford, 1990; Todd, 1959 ) and for resolution of depth layered structure, Schlumberger array is preferred over other arrays. Vertical electric soundings were carried out at 5 locations with the aid of ABEM Terrameter, SAS 300C with the SAS 2000 booster using the Schlumberger electrodes configuration. The Garmin II Plus GPS (Global Positioning System) was used to establish co-ordinate references and altitude at the VES stations. The data obtained from the field survey were processed to obtain the apparent resistivity using the geometrical factor of the Schlumberger array and the resistant obtained for each current electrode spacing (AB/2) (Dobrin, 1988). The data obtained were interpreted quantitatively and qualitatively. The plotted curve is partially curvematched with a set of two layer Schlumberger master curves in conjunction with auxiliary curve (Orellana and Mooney, 1972). The output was now passed through computerized program based on iterative method of Gauss to reduce the percentage error to the barest minimum. RESULTS A D DISCUSSIO S The processed data were used in plotting the resistivity curves (Keckler, 1995), generating the geophysical maps and the geoelectric sections. The curves obtained from the iteration are given in figures 2a 2e with VES numbers as indicated, (See Appendix) Visual inspection of the curves revealed that VES location points 1 and 3 are similar as they are of QHA-curve type with characteristic, ρ 1 >ρ 2 >ρ 3 <ρ 4 <ρ 5 and consisting of five layers. While layer 1 of VES 1 with thickness 9.5 m and resistivity value of Ωm can be said to be composed of sand, the second and third layer is seen as sandy clay and clayey sand formations which are water bearing and can serve as good regions for obtaining groundwater as it has an overburden thickness of 34 m. VES 3 with same curve type as VES 1, however, has hardpan sand in layers 1 and 2, sandy clay with saline water intrusion due to a very low resistivity value of 60.3Ωm in layer 3. VES location points 2 and 5 have similar characteristics namely: ρ 1 <ρ 2 >ρ 3 >ρ 4 <ρ 5. In VES 2, the formation has low resistivity values in layers 1, 3 and 4 and high values of 3737Ωm and Ωm in layers 2 and 5 respectively. The sharp increment from 811.3Ωm in layer 1 to 3737Ωm in layer 2 indicates the presence of clay which can serve as a confinement for the aquifer. VES point 5 has relatively resistivity in all its layers especially at layer 4 with resistivity value of 46.8Ωm, which may be due to the presence of saline water. The intrusion of saline water observed in layer 4, requires that for any borehole drilling or construction, an overburden of 20m should be considered favorable. VES 4 has HAA-curve type, five (5) layers and its resistivity character is ρ 1 >ρ 2 <ρ 3 <ρ 4 <ρ 5. The high resistivity in layers 1 and 4 which may be attributed to the presence of sand which is a water bearing formation, while layers 2 and 3 have relatively low resistivity with much content of clay (clayey sand) in layer 2 and more of sand (sandy clay) in layer 3. In this formation, in order not to have groundwater deterioration, an aquifer could be reached with an overburden thickness of 12m where we have a region of very high resistivity in the neighbourhood of 1130Ωm. Geoelectric Sections The result of vertical electrical sounding (VES) curves is presented as geoelectric sections along profiles XX XX and YY YY and was carried out to observe the variations in the character and behaviour of the sedimentary aquifer. Geoelectric Section along XX-XX' This section is taken on the South-North direction of the study area. It comprises of VES locations 3 and 1 and shown as the geoelectric section along the XX- XX' in figure 3a. The top layer which is sandy has resistivity values ranging from Ωm Ωm. The second layer has a very low resistivity value of 60.3Ωm with thickness 6.4m and depth 10.8m which is an indication of saline intrusion. The third layer is sandy with a reasonable thickness of 6.3m which could be seen as a water bearing zone. The last layer in VES 3 has a very high resistivity value of Ωm which can serve as an aquifer, since the high value signifies the presence of hard pan sand. Figure 3a: Geoelectric section along XX-XX' Geoelectric section along YY-YY' 674

3 This section is oriented along West East direction and consists of VES points 1 and 4. From the geoelectric section analysis, one could see that VES 1 is of five (5) layers where layers 1 and 5 have in common same subsurface materials. Also, layers 2 and 4 have the same subsurface entities. The first layer is sandy because of the high resistivity value ranging from Ωm Ωm with thickness ranging from 1.0m 9.5m. The second layer is of sandy clay with resistivity values ranging from Ωm 933.7Ωm and of depth 18.1m from the uppermost later. The third layer is seen to be clay with little of sand in composition (Clayey sand), of resistivity value of 446.0Ωm and thickness of about 31.0m. The eastern part of the area has five (5) layers also but with layers 1, 3 and 5 having the same subsurface composition. Sand was also the first material that existed in the eastern part of the study area at VES point 4 but it was of resistivity value of Ωm with depth 1.8m, compared to its depth in the western part of the area in VES point 1. The second layer has a resistivity value of 180.2Ωm which indicates a change in subsurface material, more of sand mixed with clay. With the alternate high and low resistivity value variations in this area, one can say that the formation would have extended downward to the last layer which is sandy and free of saline intrusion free as its resistivity value is very high with overburden thickness of 13.6m. This can serve as a region where groundwater quantity and quality will be appreciable. See Figure 3b Fig. 3b Geoelectric section along YY-YY' Table one shows the summary of the geoelectric parameters, which could also serve as a qualitative interpretation to the study Table 1: Summary of the geoelectric parameters VES o of Curve Character & Resistivity Thickness (m) Depth (m) Lithological unit o Layers Type (ohms-m) 1 5 ρ 1>ρ 2>ρ 3<ρ 4<ρ Sand QHA Sandy clay Clayey sand Sandy clay Sand 2 5 ρ 1<ρ 2>ρ 3>ρ 4<ρ Sandy clay KQH Clay Sandy clay Clay/Saline water Clayey sand 3 5 ρ 1>ρ 2>ρ 3<ρ 4<ρ Sand QHA Sand Sandy clay with saline intrusion Sand Hard pan sand 4 5 ρ 1>ρ 2<ρ 3<ρ 4<ρ Sand HAA Clayey sand Sandy clay Sand Sand(Hard pan) 5 5 ρ 1<ρ 2>ρ 3>ρ 4<ρ Clay KQH Clayey sand Clayey sand Sand / Saline intrusion Sandy clay 675

4 GEOPHYSICAL MAPS Isoresistivity Maps These are maps depicting resistivity distribution within the study area. Four maps at electrode separation of 6m, 15m, 40m, and 100m in conjunction with the 3-D view of their subsurface layers and an Isopach contour map of the study area were generated. Figure 4a shows the isoresitivity map at electrode separation of 6m. The map has a resistivity range between Ωm in the southern part. The contours are centrally located and evenly spaced, indicating that the surface is probably gentle, it also forms a closure at the southern position of the map. The area depicts a low resistivity towards the northwest part with range of 0-500Ωm because of the systematic decrease in contour values outwardly. This indicates that this area has shallow water with no element of saline intrusion. The Isoresistivity map and contour at electrode separation of 15m is shown in figure 4b. The map has a resistivity range between Ωm, with high values towards the central part ranging between 1800 and 2400Ωm. The contours are reducing towards the north and increasing towards the south-west, and are sparsely distributed. This northern part is scattered but it s compacted towards the south. This depicts low resistivity values towards the north and east with a range of Ωm because of the methodical decrease in contour value outwardly. This also indicates that the area has groundwater around the center but probably with saline intrusions around the north. Figure 4a: Isoresistivity Map and 3-D contour at AB/2 = 6m Meanwhile, the Isoresistivity map and contour at electrode separation of 40m (shown in figure 4c) revealed a resistivity range of Ωm while the separation increases towards the north with a range of Ωm. The contours are evenly spaced towards the north indicating that the surface is probably gentle, while the contours forms a closure towards the southern part with a range of Ωm because of the decrease towards the southern part. This indicates that this part has saline intrusion towards the southern part. Figure 4d depicts the Isoresistivity map and contour at electrode separation of 100m. The map has a resistivity range of Ωm, high resistivity values at the southern part of the map with the range of Ωm. The contour is compacted in the southern part of the map indicating that this environment has very high resistivity values towards the south and that shows that appreciable groundwater could be obtained around the southern part of the area. 676 Figure 4b: Isoresistivity Map and 3-D contour at AB/2 = 15m

5 depth ranges from 10-36m with a contour separation of 2m. At lower depths, the resistivity values are low indicating saline intrusion. However, at 30m, the soil has an aquifer due to higher value of resistivity. The implication is that the deeper the well for groundwater exploration, the safer it is from saline water. Figure 4c: Isoresistivity Map and 3-D contour at AB/2 = 40m Figure 4e: Isopach contour Map of the study area Figure 4d: Isoresistivity Map and 3-D contour at AB/2 = 100m Isopach Contour Map of the Study Area This map shown in fig 4, shows overburden thickness contour of depth to the bottom layer in meters. The CO CLUSIO S A total of five (5) vertical electrical soundings (VES) and geo-electric sections were carried out and interpreted. The field data were gotten from Schlumberger electrode configuration and the apparent resistivities were obtained. The data revealed sand, clay, sandy clay, clayey sand and some intrusions of saline water. Also the geophysical parameters of the subsurface units such as the top layer resistivity and thickness of the bed boundaries and correlation of such beds were determined. The subsurface integrity determination and characterization is important and a pre-requisite before any groundwater exploitation. This study gives reliable and consistent information on subsurface layering and lithologies and also revealed that most of the points are good sites for borehole construction in this coast-line area of Lagos state. However, to avoid the deterioration in the quantity and quality of water from such boreholes in the area, a deeper depth and a very thick bed are required REFERE CES Dobrin, M.B. (1988): Introduction to Geophysical Prospecting 4 th edition, McGraw-Hill companies, Inc. New York. 677

6 Keckler, D.(1995): Contouring and 3D Surface Mapping System, Surfer Version 6, Golden Software Inc. Colorado, USA. Lowrie. W. (1997): Fundamentals of Geophysics Cambridge University Press, United Kingdom Orellana and Mooney (1972): Two and three layer master curve and auxiliary point diagram for VES (1972) Madrid, Interciencia. Oseni, I.O. (2005). Geophysical and Geotechnical Evaluation of Foundation Conditions in Lagos area, South Western Nigeria. Unpublished B.Sc. Thesis, Department of Physics, Olabisi Onabanjo University Ago-Iwoye, Nigeris. Telford, W.M, Geldart L.P and Sherif R.E (1990), Resistivity Methods, Applied Geophysics, Cambridge University Press, United Kingdom. pg Todd, D.K. (1959): Groundwater Hydrology Second edition, John Wiley and sons Inc New York, pg 336. APPE DIX 678

7 Figure 2e: Resistivity curve for VES point 5 679